Intumescent coating compositions are well-known in the art. An outstanding feature of intumescent coatings is that they may be applied on substrates, such as metal, wood, plastics, graphite and other materials, in the manner of a coating having relatively low film thickness. Upon exposure to fire, heat or flames, the intumescent coatings expand considerably in terms of thickness to produce an insulative layer of char and char foam.
The most commonly used intumescent coatings contain four basic components, sometimes called “reactive pigments”, dispersed in a binder matrix. The reactive pigments include (1) an inorganic acid or a material which yields an acid at temperatures between 100° C. and 250° C., such as for example, ammonium polyphosphate which yields phosphoric acid; (2) a carbon source such as a polyhydric material rich in carbon, also referred to as a carbon hydrate, for example, pentaerythritol or dipentaerythritol; (3) an organic amine or amide, such as for example, a melamine; and optionally (4) a halogenated material which releases hydrochloric acid gas on decomposition.
The basic intumescent mechanism is proposed to involve the formation of a carbonaceous char by the dehydration reaction of the generated acid with the polyhydric material. The amine may participate in char formation, but is described primarily as a blowing agent for insulating char foam formation. Because the insulating char stops fire and remains on the substrate, it offers better fire and thermal protection under severe fire conditions than non-flammable type coatings.
Numerous patents and publications, have disclosed intumescent compositions containing one or more polymeric materials in combination with phosphate containing materials and carbonific or carbonic yielding materials.
In European Patent 0 902 062, the intumescent coating compositions can comprise vinyltoluene/acrylate copolymers or styrene/acrylate polymers as a film-forming binder.
In U.S. Pat. No. 3,654,190, the intumescent coating contains a solid vinyltoluene/butadiene copolymer associated to a chlorinated natural rubber acting as a char former.
In European Patent 0 342 001, polymeric binder for intumescent coatings comprise copolymers formed of a first monomer in a predominant amount and of a second monomer in a minor amount, said second monomer being a thermally labile co-monomer which is preferably a monomeric aledhyde such as acroleine.
In WO Patent 01/05886, a polymeric binder in an emulsion form is operative to form a film when the composition is allowed to dry. The polymeric binder described by WO Patent 01/05886 is a styrene/acrylate copolymer.
The coatings industry seeks fire retardant coatings which not only meet fire retardancy requirements, but which also possess desirable coating properties. The reactive pigments utilised in the formulation of an intumescent coating are not sufficient in and of themselves to provide desirable coating properties. For example, an intumescent coating must provide all the performance characteristics expected of a conventional coating plus the added benefit of fire retardancy. Incorporating both fire retardance and good coating properties in one system is not straightforward. The combinations of additives, such as, for formulating an intumescent coating can often result in a formulation possessing both poor coating and poor fire retardancy properties.
It was found that the chemical and physical properties of the binder are critical to the functioning of an intumescent coating. On one hand, the binder should not soften or melt too quickly to permit the formation of a stable char. On the other hand, the viscosity of the binder is correlated with the diffusion and the char formation.
It is therefore desired to provide a polymer binder for intumescent coatings which reduces flame spread during the early stages of a fire and which contributes to improve the char formation and intumescence during the latter stages of the fire.